The present invention relates to a method of separating and recovering each component from a mixed solution of organic solvents obtained during the production of .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as ".alpha.-APM"), which is useful as a sweetener, namely, a mixed solution of acetic acid and toluene or acetic acid, toluene and formic acid. A mixed solution obtained during the production of .alpha.-APM is generally composed of 15 to 45% by weight of acetic acid and 50 to 80% by weight of toluene in the former case, and 15 to 50% by weight of acetic acid, 40 to 80% by weight of toluene and 1 to 15% by weight of formic acid in the latter case. Each component separated and recovered from the mixed solution can be recycled to an .alpha.-APM producing process and reused.
Since acetic acid (boiling point: 118.degree. C.) and toluene (boiling point: 111.degree. C.) form an azeotropic mixture (azeotropic point: 104.degree. C.) containing 34.5% by weight of acetic acid, it is theoretically impossible to separate the mixed solution by a single and simple distillation operation and obtain acetic acid containing substantially no toluene and toluene containing substantially no acetic acid, simultaneously.
In a usual separating method, acetic acid in toluene is first extracted with water, an extracting solvent, into the water phase and the obtained mixed solution of water and acetic acid are then separated by distillation. Separation of water and acetic acid by a conventional distillation method requires as many as 80 to 90 plates and a high reflux ratio. Therefore, azeotropic distillation using an azeotropic solvent (hereinafter referred to as "entrainer"), such as butyl acetate, is generally used so as to obtain a mixed solution of water and the entrainer from the top of the column and obtain acetic acid from the bottom of the column.
In the method, two separating operations, namely, extraction and distillation, are carried out sequentially, and each operation requires its own apparatus. Especially, when the concentration of acetic acid in the mixed solution is high, in order to extract almost all part of the acetic acid into water phase, an extraction column having a large number of plates such as 20 to 30 becomes necessary and the whole equipments including the extraction column in addition to the distillation column become excessively large. Since the production of .alpha.-APM requires a large number of solvent recovering processes, it is desirable that the number of operations necessary in each process is as small as possible and that the equipment in each operation is as small as possible.
A technique to simultaneously isolate three components in a mixed solution of acetic acid, toluene and formic acid (boiling point: 101.degree. C.) into each component in a single operation has not been known. However, a technique to separate components in a mixed solution of formic acid and acetic acid by azeotropic distillation adding toluene as an entrainer by utilizing the fact that formic acid and toluene mixed in a weight ratio of 50:50 forms a two-liquid-phase azeotropic mixture (azeotropic point: 86.degree. C.) and to recover formic acid from the top of the distillation column and acetic acid from the bottom of the column is known. In this case, since toluene is not contained in the original mixed solution which is the object of the separation, usually all the toluene phase distilled out at the top of the column is refluxed and only an amount of toluene, being dissolved in the formic acid phase discharged out of the distillation system, is supplied to make up for the discharged amount. However, the mixed solution which is obtained during the production of .alpha.-APM originally contains toluene. Therefore, when this known technique is applied to separate the mixed solution, containing toluene, the amount of toluene to be supplied to the distillation system must be taken out and recovered from the toluene phase distilled out at the top of the column. Namely, each component of the mixed solution must be separated, by (i) recovering acetic acid from the bottom of the column, (ii) recovering a mixed solution of formic acid and toluene from the top, (iii) making the mixed solution into two liquid phases, (iv) separating the layers of toluene and formic acid, and (v) recovering each component.
However, the concentration of toluene in the mixed solution obtained during the production of .alpha.-APM is as high as 5 to 25 times by weight the concentration of formic acid and a mutual solubility of formic acid and toluene is as high as 8% by weight at 25.degree. C., so if the mixed solution is separated by this conventional method, a considerably large part of formic acid dissolves in the recovered toluene phase. Since the recovered toluene is neutralized and washed, formic acid in the recovered toluene phase is wasted in the form of an aqueous diluted solution. Accordingly, the larger the amount of formic acid in the recovered toluene phase is, the larger loss of formic acid becomes and, as a result, the recovery yield of formic acid becomes poor.
For example, when separating a mixed solution of 7% by weight of formic acid, 25% by weight of acetic acid and 68% by weight of toluene by this conventional method, as much as 84% of the formic acid in the mixed solution supplied to the distillation system is taken out dissolving in the recovered toluene phase. Because this part of the acid becomes a waste, the recovery ratio of formic acid becomes as low as 16%. From the point of view of the profitability of the separating and recovering process, the desirable recovery ratio of formic acid is not less than 90%, but that of formic acid obtained by this conventional method falls far below the value.
Consequently, when the recovery of formic acid is taken into consideration, this conventional technique cannot be applied to the process of producing .alpha.-APM.
The present inventors have intensively studied to solve the above-described problems and to establish a method of separating and recovering each of the components from a mixed solution of acetic acid and toluene or a mixed solution of acetic acid, toluene and formic acid with high recovery ratios.
As a result of the studies carried out by the present inventors to solve the problems, it has been found that it is possible to separate each of the components from a mixed solution of acetic acid and toluene in a single operation by azeotropic distillation using water as an entrainer with high recovery ratios. Further, it has also been found that it is possible to separate and recover each of the components from a mixed solution of acetic acid, toluene and formic acid only by two distilling operations, namely; (a) distilling the mixed solution and recovering formic acid from the top and a mixture of acetic acid and toluene from the bottom; and (b) distilling the mixture of acetic acid and toluene using water as an azeotropic solvent and recovering toluene from the top and acetic acid from the bottom. Based on the two findings described above, the present inventors have attained the present invention.